This invention relates to laser resonators and in particular it relates to the balancing of laser resonators for maximum efficiency. The invention relates specifically to a multi-element laser such as a multi-rod laser.
A multi-rod laser is described in commonly-assigned pending U.S. application Ser. No. 08/082,769 which is incorporated herein by reference. The laser is briefly described below for illustrative purposes only. The invention is also applicable to other laser systems.
Referring now to FIG. 1, a laser system comprises an optical head 12 and a power supply 14 controlled by a computer 16. The optical head 12 has a pair of mirrors 30, 32 which together define an optical resonator. Inside this optical resonator, there are four identical pumping sections or chambers 34.
Each pumping section 34 comprises a pair of discharge lamps 36 which provide pumping light to a laser element 38 (in this case, a solid state laser element). In each pumping chamber 34 the two lamps 36 and the laser element 38 are surrounded by a reflector. The laser element 38 is also surrounded by a water jacket. In operation, filtered, deionized cooling water passes through the water jacket.
The laser element 38 is a solid rod of active laser medium in the form of YAG doped with neodymium. Each laser rod has concave, anti-reflection coated end faces. As is well known, Nd:YAG emits light at a wavelength of 1064 nm. Nd:YAG represents only one example of active laser media which may be used with the present invention. The active laser medium may emit light either inside or outside the visible spectrum. The invention is also applicable to the operation of Nd:YAG at wavelengths other than 1064 nm.
As shown in FIG. 2 each lamp 36 comprises a pair of electrodes 46 sealed inside a quartz envelope or tube 48. In order to support a discharge between the electrodes 46, the quartz tube 48 contains krypton gas. The lamps 36 are energized by a power source.
The mirrors 30, 32 provide positive feedback and are plane parallel mirrors formed from coated quartz. On its side facing towards the middle of the optical resonator, the mirror 30 has a reflectivity in excess of 99%. The mirror 32 is the output mirror of the optical head 12 and, on its side facing the middle of the optical resonator, it has a coating that is partially reflective at the laser wavelength.
In each pumping chamber 34, one electrode 46 of one lamp is connected to one electrode 46 of the other lamp 36 so that the lamps 36 are connected in series. The power supply 14 is connected to the other electrode 46 of each lamp 36. Thus, the power supply 14 supplies current to the two electrodes 46 of each lamp 36. The pumping chambers are equally spaced. Furthermore, the laser mirrors are positioned so that the resonator is `symmetric`.
In general terms, laser oscillation in the system shown in FIG. 1 occurs by virtue of the laser elements 38 providing an active laser (ie gain) medium and the mirrors 30 and 32 providing positive feedback. Oscillation between the two mirrors occurs in one or more stable modes. These are described in "Solid-state laser engineering", Walter Koechner, page 189 et seq, Springer-Verlag, 1992. In particular, this reference describes how a laser resonator can be either stable or unstable. The stability of the resonator is determined by the optical properties (e.g. focal length) of the resonator mirrors and of the laser medium inside the resonator. Occasionally, unstable resonators are used to obtain good laser beam quality but these are difficult to control and very sensitive to changes. Usually, it is required to obtain a stable resonator. It should be noted that the optical characteristics of laser media tend to change with input power and thus the stability may also change when power is changed.
It is common to describe the stability of a resonator in terms of parameters g1 and g2 as described on page 200 of this reference. The so-called g parameters are determined by the focal lengths of, and the distances between the optical components inside the resonator. It is found that some combinations of g1 and g2 result in stability whilst others do not. This is usually illustrated graphically by drawing the g-plane which has stable and unstable regions. A resonator with fixed g parameters is represented by a point in this plane. When the optical characteristics of the resonator are subject to change, the point moves along a trajectory through the plane and may move in and out of stability regions.
It is often required to use resonators with more than one laser medium region inside the resonator, to increase power. The laser rods used in such lasers are very prone to thermal lensing effects caused by heat input from the discharge lamps, which means that their focal length changes dramatically with lamp input power (and so too with laser output power). This leads to very pronounced movements of the stability point in the g-parameter plane. This effect is described more fully in "Multi rod resonators for high-power solid-state lasers with improved beam quality", K. P. Driedger et al, IEEE Journal of Quantum Electronics, Vol. 24. No. 4, April 1988, pages 665 to 674.
The most practical multi-rod arrangement is the so-called symmetric (or periodic) resonator. This tries to minimise the extent of incursions into instability regions by imposing periodic symmetry onto the resonator. In the symmetric resonator, if the center-line spacing of the pumping chambers is 2L, then the plane mirrors are positioned a distance L from the respective center-line of the adjacent pumping chambers. Moreover, for perfect symmetry, the focal length of the laser rods must be identical at all times. It is, however, generally acknowledged that a perfectly symmetric resonator is impossible to obtain since the thermal and optical characteristics of all the rods in a multi-rod laser system cannot be perfectly matched in all aspects. Therefore, unstable operating regions still occur and have up to now been tolerated. In these regions, beam characteristics deteriorate dramatically, thus degrading the usefulness of such laser systems.
Unstable regions in laser resonators principally reveal themselves as dips in a graph when laser output power is plotted as a function of input power to the discharge lamps. A schematic example of this is shown in FIG. 3 which illustrates how, as the input power to the lamps increases, the output power generally increases apart from at unstable regions where dips in the output power occurs. There may be only one dip or a plurality of dips may occur. This effect is well documented.
Up to now, attempts to overcome these instabilities have involved careful design, and matching of the various laser elements. Resonators have been used which are as near symmetric as possible; in particular laser rods have been spaced as carefully as possible. Other than that, it has been accepted that little can be done other than by trying to select laser rods with similar characteristics.
In 1992, M Kumkar et al conceded in Optics & Laser Technology Vol 24 No 2 1992, pages 67 to 72, that different refractive powers of individual laser rods cannot be avoided. Also, in 1993, Goeller et al reported results for a state-of-the-art project in Laser und Optoelektronik Vol 25 No 2, 1993, pages 42 to 46, that exhibited power dips without any proposals for solutions to the problem.
It is an object of the present invention to attempt to improve laser resonators by reducing or eliminating the presence of unstable regions in the power output curve.
It is further an object of the present invention to provide a method for reducing instabilities in output power of a multi-element laser.
Other and further objects, advantages and features of the invention will become apparent.